The major objectives of this project continue to center on defining the molecular mechanism(s) of recombination in the model eucaryote Saccharomyces cerevisiae. Though our knowledge of recombination is at best partial and incomplete, we sense rather wide-spread agreement with our notion that gene conversion associated exchange alone can account for the total meiotic map particularly in a system amenable to the large scale analysis of unselected tetrads. Conceived at the genetic level and utilizing single cell analysis, these ongoing studies address such issues as: what are the principal features of heteroduplex formation and heteroduplex repair in meiotic and mitotic cells. The major goals and experimental programs for the current period include: the further development and subsequent exploitation of a system selective for mutants deficient in the excision-resynthesis reaction or a process which presumably occurs subsequent to the production of meiotic heteroduplex DNA. The rationale was set forth in detail in the 1978 presentation. Strains have been collected and the initial hybrids synthesized and tested. The large scale analysis of unselected tetrads obtained from a diploid heterozygous for three maximally separated sites within the arg4 locus, i.e., (3 plus 36 over plus 16 plus) is in progress. To date we have demonstrated that all possible allelic recombinants and post-meiotic segregants are uniquely and unambiguously identifiable by our in situ allele identification procedures based on complementaion and suppression. Outside marker recombination, exact position of the associated exchange and interference (chiasma and chromatid) will be monitored. Special attention will be given to the effect of intragenic heterozygosities on the corerction of a centrally located DNA heteroduplex segment.